Benzoxazines and compositions containing benzoxazine are known (see, for example, U.S. Pat. Nos. 5,543,516 and 6,207,786 to Ishida et al.; S. Rimdusit and H. Ishida, “Development of New Class of Electronic Packaging Materials Based on Ternary Systems of Benzoxazine, Epoxy, and Phenolic Resins”, Polymer, 41, 7941-49 (2000); and H. Kimura et al., “New Thermosetting Resin from Bisphenol A-based Benzoxazine and Bisoxazoline”, J. App. Polym. Sci., 72, 1551-58 (1999).
U.S. Pat. No. 7,517,925 (Dershem et al.) describes benzoxazine compounds and thermosetting resin compositions prepared therefrom. The compositions are said to be useful for increasing adhesion at interfaces within microelectronic packages and low shrinkage on cure and low coefficient of thermal expansion (CTE).
U.S. Pat. No. 7,053,138 (Magendie et al.) describes compositions comprising benzoxazines and thermoplastic or thermoset resins in the manufacture of prepregs and laminates. The compositions are said to yield flame-proofed laminating resins that have high glass transition temperatures.
U.S. Pat. No. 6,376,080 (Gallo) describes a method of preparing a polybenzoxazine which includes heating a molding composition including a benzoxazine and a heterocyclic dicarboxylic acid to a temperature sufficient to cure the molding composition, thereby forming the polybenzoxazine. The compositions are said to have near-zero volume change after post cure.
U.S. Pat. No. 6,207,786 (Ishida et al.) states that the polymerization of benzoxazine monomers to a polymer is believed to be an ionic ring opening polymerization which converts the oxazine ring to another structure, e.g., linear polymer or larger heterocyclic rings. It is thought that a chain transfer step(s) limits the molecular weight of the resulting polymer and causes some branching. FTIR (Fourier transform infrared) analysis is often used to monitor the conversion of the oxazine rings to polymers to provide an estimate of the rate of polymerization at different temperatures. NMR (nuclear magnetic resonance) spectroscopy can also be used to monitor conversion of benzoxazine monomers to polymer.
Epoxy adhesives have been widely used in structural adhesive applications and satisfy many demanding industrial applications. However epoxies have many noted deficiencies that limit their use including limited high temperature stability, high moisture uptake, shrinkage, and a large exotherm on polymerization.
Polybenzoxazines have been proposed to overcome many of the limitations on epoxies. They have lower exotherms on curing, less shrinkage, have higher thermal stability, low byproducts and may be readily prepared from benzoxazines, which in turn, are readily prepared from an amine, formaldehyde and a phenol in high yields. However, current methods of preparing polybenzoxazines require relatively high temperatures, and typically produce brittle, highly crosslinked polymers.
Efforts to reduce the polymerization temperature have included the addition of various phenols or Lewis acid accelerators, or copolymerization of the benzoxazine with epoxides or other monomers such as phenol-formaldehyde. However the resultant polybenzoxazines-epoxy hybrids retain many of the limitations of the epoxies, and compromise many desirable features thereof, such as epoxy toughness.